A device in which mechanical power is converted to fluid power, and the fluid power is returned to the mechanical power to perform power transmission is called a hydraulic power transmission. A fluid coupling is a kind of hydraulic power transmission. Examples of the fluid coupling include those having various structures and actions. A viscous coupling is used in a power transmission device for a differential limiting-device for automobile, a differential gear for four-wheel drive car or a cooling fan for an automobile engine, or the like.
The viscous coupling is a device in which disks (plates) or cylinders separately connected to input and output shafts are arranged in such a manner that gaps therebetween are sufficiently narrow, and power is transmitted by shearing force based on the viscosity of a fluid in the gaps.
The viscous coupling is a sort of liquid clutch, which permits smooth slide. A typical specific structure thereof is constructed in such a manner that plural inner plates arranged movably on the side of a drive shaft (input shaft) and plural outer plates fixed on the side of a driven shaft (output shaft) are alternately combined with each other, and individual gaps between the alternately combined plates are held at regular intervals by spacers such as separate rings. These plates are contained in a housing in which a viscous fluid for transmitting torque is filled. The viscous fluid is filled in the spaces between the plural plates.
The viscous coupling servers to generate viscous torque in the spaces between the plates when a difference in revolution speed between the drive shaft and the driven shaft arises, and torque is transmitted on the side of the driven shaft in proportion to the viscous torque generated owing to the difference in revolution speed.
As the viscous fluid, silicone oil is generally used. Specifically, polyorganosiloxanes such as dimethyl polysiloxane (i.e., dimethyl silicone oil) and methylphenyl polysiloxane (i.e., methylphenyl silicone oil) are used as the silicone oil. These polyorganosiloxanes are good in heat resistance and oxidation resistance compared with other base oils and moreover in temperature-viscosity characteristics over a wide range and have a high viscosity index (VI).
However, since the temperature of the oil is raised to about 100.degree.-180.degree. C. according to the service conditions of the viscous coupling, or to such a high temperature as exceeding 200.degree. C. under severe conditions, for example, such as repeated hump-stack, the stability of the polyorganosiloxane is lowered, and so abnormal wear of the plates and gelation of the polyorganosiloxane occur. The gelation of the polyorganosiloxane is considered to increases its viscosity because a polymerization reaction occurs on the polymer. Accordingly, its viscosity stability is also impaired in association with the gelation.
As described above, the polyorganosiloxanes are low in stability at a high temperature and are hence difficult to stably keep the torque-transmitting performance over a long period of time under severe service conditions. As a countermeasure, it has heretofore been proposed to incorporate various additives such as an antioxidant and an extreme-pressure additive.
For example, Japanese Patent Application Laid-Open No. 65195/1989 has proposed a fluid composition for a viscous coupling in which a specific sulfur compound or a metal salt of dialkyldithiocarbamic acid is incorporated into a polyorganosiloxane. Japanese patent Application Laid-Open No. 91196/1990 has proposed a fluid composition for a viscous coupling in which a specific phosphorus compound is incorporated into a polyorganosiloxane. Japanese patent Application Laid-Open No. 269093/1991 has proposed a fluid composition for a viscous coupling in which a metal deactivator is incorporated in a proportion of 0.01-1.0 wt. % into a polyorganosiloxane. In Japanese patent Application Laid-Open No. 50296/1992, it has been proposed to add a metal deactivator and/or a corrosion inhibitor to a polyorganosiloxane.
However, these conventional compositions have not been yet fully satisfactory in anti-gelling performance, viscosity stability and torque stability.